Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536801010583/cv6028sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536801010583/cv6028Isup2.hkl |
CCDC reference: 170884
The title compound was synthezized according to a known procedure (Arrieta, 1993). The crystals grown by cooling a saturated warm solution in toluene have a melting point of 318 K. The IR spectra were obtained from a Nicolet 205 F T–IR spectrometer using KBr pellet technique as well as liquid solution (0.4656 M, δ = 2.2 mm) The NMR spectra (1H and 13C) were recorded on Bruker instruments AC 250 and ARX 300. The signals δ = 7.26 p.p.m. 1H and δ = 77.7 p.p.m. 13C for CDCl3 were selected as internal standards. Two characteristic IR absorption bands at 1698 and 1624 cm-1 with about equal intensities (in KBr) were assigned to the non coordinated carbethoxy group and the cinnamonyl group, in coordination with the acyl group, respectively. In chloroform solution (0.47 M), the first band is shifted to 1701 cm-1 (ε = 39 l mol-1 cm-1 and the chelate band to 1630 cm-1 (ε = 44 l mol-1 cm-1. In KBr and CHCl3, a compared profile was found for the bands at 1268 and 1308 cm-1 (KBr) and 1278 and 1308 cm-1 (CHCl3).
Data collection: SMART (Bruker, 1998); cell refinement: SMART; data reduction: SAINT (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXL97; software used to prepare material for publication: SHELXL97.
C15H16O4 | Dx = 1.257 Mg m−3 |
Mr = 260.28 | Melting point: 45 K |
Orthorhombic, Pnn2 | Mo Kα radiation, λ = 0.71073 Å |
a = 21.698 (4) Å | Cell parameters from 1024 reflections |
b = 7.3390 (15) Å | θ = 3.7–26.3° |
c = 8.6380 (17) Å | µ = 0.09 mm−1 |
V = 1375.5 (5) Å3 | T = 123 K |
Z = 4 | Needle, yellow |
F(000) = 552 | 0.4 × 0.3 × 0.3 mm |
Bruker SMART diffractometer | 2813 independent reflections |
Radiation source: fine-focus sealed tube | 2703 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.024 |
Detector resolution: 8 pixels mm-1 | θmax = 26.4°, θmin = 2.5° |
ω scans | h = −27→26 |
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996) | k = −9→9 |
Tmin = 0.964, Tmax = 0.973 | l = −10→10 |
13464 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.031 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.079 | All H-atom parameters refined |
S = 1.06 | w = 1/[σ2(Fo2) + (0.038P)2 + 0.275P] where P = (Fo2 + 2Fc2)/3 |
2813 reflections | (Δ/σ)max = 0.001 |
236 parameters | Δρmax = 0.13 e Å−3 |
1 restraint | Δρmin = −0.16 e Å−3 |
C15H16O4 | V = 1375.5 (5) Å3 |
Mr = 260.28 | Z = 4 |
Orthorhombic, Pnn2 | Mo Kα radiation |
a = 21.698 (4) Å | µ = 0.09 mm−1 |
b = 7.3390 (15) Å | T = 123 K |
c = 8.6380 (17) Å | 0.4 × 0.3 × 0.3 mm |
Bruker SMART diffractometer | 2813 independent reflections |
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996) | 2703 reflections with I > 2σ(I) |
Tmin = 0.964, Tmax = 0.973 | Rint = 0.024 |
13464 measured reflections |
R[F2 > 2σ(F2)] = 0.031 | 1 restraint |
wR(F2) = 0.079 | All H-atom parameters refined |
S = 1.06 | Δρmax = 0.13 e Å−3 |
2813 reflections | Δρmin = −0.16 e Å−3 |
236 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
C4 | 0.54666 (6) | 0.30193 (18) | 0.76650 (17) | 0.0281 (3) | |
O1 | 0.43932 (5) | 0.18041 (16) | 0.90040 (12) | 0.0424 (3) | |
O2 | 0.54340 (5) | 0.29967 (15) | 0.91770 (12) | 0.0377 (2) | |
C2 | 0.44083 (6) | 0.18612 (18) | 0.75416 (17) | 0.0303 (3) | |
C3 | 0.49602 (6) | 0.24780 (18) | 0.67582 (18) | 0.0267 (3) | |
C6 | 0.65241 (6) | 0.41141 (19) | 0.79130 (17) | 0.0311 (3) | |
C5 | 0.60448 (6) | 0.36803 (18) | 0.70073 (16) | 0.0290 (3) | |
C7 | 0.71294 (7) | 0.47571 (18) | 0.73952 (18) | 0.0319 (3) | |
C1 | 0.38516 (7) | 0.1190 (2) | 0.6689 (2) | 0.0358 (3) | |
C10 | 0.82952 (8) | 0.6003 (2) | 0.6492 (3) | 0.0489 (4) | |
C8 | 0.72895 (8) | 0.4860 (3) | 0.5835 (2) | 0.0445 (4) | |
C12 | 0.75629 (8) | 0.5312 (2) | 0.8491 (2) | 0.0435 (4) | |
C9 | 0.78637 (9) | 0.5492 (3) | 0.5391 (3) | 0.0557 (5) | |
C11 | 0.81407 (8) | 0.5933 (3) | 0.8036 (3) | 0.0508 (5) | |
O4 | 0.44910 (4) | 0.31302 (13) | 0.43790 (11) | 0.0308 (2) | |
O3 | 0.54393 (4) | 0.19264 (14) | 0.43011 (12) | 0.0344 (2) | |
C14 | 0.44371 (7) | 0.2908 (2) | 0.27055 (18) | 0.0345 (3) | |
C13 | 0.50018 (6) | 0.24626 (17) | 0.50386 (17) | 0.0260 (3) | |
H8 | 0.6985 (12) | 0.452 (3) | 0.501 (3) | 0.074 (7)* | |
H5 | 0.6058 (7) | 0.377 (2) | 0.5859 (19) | 0.026 (4)* | |
H14B | 0.4419 (8) | 0.161 (3) | 0.250 (2) | 0.035 (4)* | |
H14A | 0.4806 (8) | 0.345 (2) | 0.223 (2) | 0.038 (4)* | |
H1C | 0.3631 (9) | 0.221 (3) | 0.625 (2) | 0.045 (5)* | |
H12 | 0.7451 (9) | 0.526 (3) | 0.951 (3) | 0.049 (5)* | |
H1A | 0.3583 (11) | 0.053 (3) | 0.735 (3) | 0.063 (6)* | |
H9 | 0.7968 (10) | 0.543 (3) | 0.419 (3) | 0.066 (6)* | |
C15 | 0.38452 (9) | 0.3814 (3) | 0.2225 (2) | 0.0466 (4) | |
H15C | 0.3873 (10) | 0.511 (3) | 0.244 (3) | 0.062 (6)* | |
H6 | 0.6492 (7) | 0.4027 (19) | 0.9022 (19) | 0.022 (3)* | |
H15B | 0.3481 (10) | 0.326 (3) | 0.281 (2) | 0.058 (6)* | |
H10 | 0.8709 (10) | 0.640 (3) | 0.617 (2) | 0.049 (5)* | |
H11 | 0.8433 (10) | 0.633 (3) | 0.882 (3) | 0.060 (6)* | |
H1B | 0.3938 (9) | 0.039 (3) | 0.582 (2) | 0.051 (5)* | |
H15A | 0.3773 (10) | 0.370 (3) | 0.101 (3) | 0.060 (6)* | |
H2 | 0.4992 (12) | 0.255 (3) | 0.938 (3) | 0.080 (8)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
C4 | 0.0276 (6) | 0.0258 (6) | 0.0309 (6) | 0.0030 (5) | 0.0003 (5) | −0.0003 (5) |
O1 | 0.0383 (6) | 0.0518 (7) | 0.0373 (6) | −0.0041 (5) | 0.0098 (5) | 0.0029 (5) |
O2 | 0.0348 (5) | 0.0486 (6) | 0.0296 (5) | −0.0033 (5) | 0.0003 (4) | 0.0004 (5) |
C2 | 0.0290 (6) | 0.0269 (6) | 0.0350 (7) | 0.0015 (5) | 0.0059 (6) | 0.0006 (5) |
C3 | 0.0257 (6) | 0.0225 (6) | 0.0320 (6) | 0.0021 (5) | 0.0022 (5) | 0.0000 (5) |
C6 | 0.0303 (7) | 0.0288 (6) | 0.0343 (7) | 0.0022 (5) | −0.0005 (6) | 0.0003 (6) |
C5 | 0.0270 (7) | 0.0267 (6) | 0.0333 (7) | 0.0015 (5) | 0.0008 (5) | 0.0009 (5) |
C7 | 0.0283 (7) | 0.0271 (7) | 0.0404 (8) | 0.0036 (5) | −0.0031 (6) | −0.0001 (6) |
C1 | 0.0278 (7) | 0.0343 (7) | 0.0452 (8) | −0.0037 (6) | 0.0057 (6) | 0.0012 (7) |
C10 | 0.0267 (8) | 0.0424 (9) | 0.0777 (12) | −0.0027 (7) | 0.0051 (8) | 0.0064 (8) |
C8 | 0.0370 (8) | 0.0527 (9) | 0.0439 (8) | −0.0039 (7) | 0.0028 (7) | −0.0077 (7) |
C12 | 0.0374 (8) | 0.0486 (10) | 0.0444 (8) | −0.0040 (7) | −0.0075 (7) | 0.0052 (7) |
C9 | 0.0439 (9) | 0.0635 (11) | 0.0596 (11) | −0.0082 (9) | 0.0156 (8) | −0.0057 (9) |
C11 | 0.0332 (8) | 0.0457 (9) | 0.0734 (13) | −0.0051 (7) | −0.0155 (8) | 0.0077 (8) |
O4 | 0.0288 (5) | 0.0349 (5) | 0.0288 (5) | 0.0034 (4) | 0.0003 (4) | −0.0018 (4) |
O3 | 0.0269 (5) | 0.0436 (6) | 0.0326 (5) | 0.0027 (4) | 0.0031 (4) | −0.0041 (4) |
C14 | 0.0350 (7) | 0.0399 (8) | 0.0286 (7) | 0.0012 (6) | 0.0001 (6) | −0.0022 (6) |
C13 | 0.0233 (6) | 0.0235 (6) | 0.0312 (6) | −0.0016 (5) | 0.0009 (5) | −0.0009 (5) |
C15 | 0.0508 (10) | 0.0535 (10) | 0.0355 (8) | 0.0143 (8) | −0.0076 (7) | −0.0009 (8) |
C4—O2 | 1.3081 (18) | C10—C11 | 1.376 (3) |
C4—C3 | 1.4066 (19) | C10—C9 | 1.386 (3) |
C4—C5 | 1.4601 (18) | C10—H10 | 0.98 (2) |
O1—C2 | 1.2643 (18) | C8—C9 | 1.384 (2) |
O1—H2 | 1.45 (3) | C8—H8 | 1.00 (3) |
O2—H2 | 1.03 (3) | C12—C11 | 1.391 (2) |
C2—C3 | 1.4481 (19) | C12—H12 | 0.92 (2) |
C2—C1 | 1.498 (2) | C9—H9 | 1.06 (2) |
C3—C13 | 1.4882 (18) | C11—H11 | 0.97 (2) |
C6—C5 | 1.3398 (19) | O4—C13 | 1.3390 (16) |
C6—C7 | 1.466 (2) | O4—C14 | 1.4594 (18) |
C6—H6 | 0.963 (16) | O3—C13 | 1.2091 (17) |
C5—H5 | 0.994 (16) | C14—C15 | 1.505 (2) |
C7—C8 | 1.394 (2) | C14—H14B | 0.972 (18) |
C7—C12 | 1.395 (2) | C14—H14A | 0.986 (18) |
C1—H1C | 0.96 (2) | C15—H15C | 0.97 (2) |
C1—H1A | 0.95 (2) | C15—H15B | 1.02 (2) |
C1—H1B | 0.97 (2) | C15—H15A | 1.07 (2) |
O2—C4—C3 | 120.67 (13) | C9—C8—C7 | 120.73 (17) |
O2—C4—C5 | 116.06 (12) | C9—C8—H8 | 118.6 (15) |
C3—C4—C5 | 123.25 (14) | C7—C8—H8 | 120.6 (15) |
C2—O1—H2 | 101.0 (11) | C11—C12—C7 | 120.78 (18) |
C4—O2—H2 | 103.2 (15) | C11—C12—H12 | 121.8 (12) |
O1—C2—C3 | 119.92 (14) | C7—C12—H12 | 117.4 (12) |
O1—C2—C1 | 117.34 (13) | C8—C9—C10 | 120.54 (19) |
C3—C2—C1 | 122.68 (14) | C8—C9—H9 | 116.7 (12) |
C4—C3—C2 | 118.30 (14) | C10—C9—H9 | 122.4 (12) |
C4—C3—C13 | 120.71 (12) | C10—C11—C12 | 120.39 (17) |
C2—C3—C13 | 120.94 (12) | C10—C11—H11 | 120.5 (13) |
C5—C6—C7 | 126.44 (14) | C12—C11—H11 | 119.1 (13) |
C5—C6—H6 | 120.6 (9) | C13—O4—C14 | 116.54 (11) |
C7—C6—H6 | 113.0 (9) | O4—C14—C15 | 106.98 (13) |
C6—C5—C4 | 121.24 (13) | O4—C14—H14B | 107.3 (11) |
C6—C5—H5 | 122.9 (9) | C15—C14—H14B | 110.3 (10) |
C4—C5—H5 | 115.8 (9) | O4—C14—H14A | 107.7 (10) |
C8—C7—C12 | 118.16 (15) | C15—C14—H14A | 113.4 (10) |
C8—C7—C6 | 122.41 (14) | H14B—C14—H14A | 110.8 (15) |
C12—C7—C6 | 119.42 (14) | O3—C13—O4 | 123.01 (13) |
C2—C1—H1C | 109.7 (11) | O3—C13—C3 | 125.16 (13) |
C2—C1—H1A | 111.5 (14) | O4—C13—C3 | 111.83 (11) |
H1C—C1—H1A | 109.1 (17) | C14—C15—H15C | 109.0 (13) |
C2—C1—H1B | 115.0 (12) | C14—C15—H15B | 110.3 (12) |
H1C—C1—H1B | 105.4 (16) | H15C—C15—H15B | 110.1 (18) |
H1A—C1—H1B | 105.9 (17) | C14—C15—H15A | 111.3 (12) |
C11—C10—C9 | 119.37 (16) | H15C—C15—H15A | 106.1 (17) |
C11—C10—H10 | 120.5 (11) | H15B—C15—H15A | 109.9 (16) |
C9—C10—H10 | 120.2 (11) | ||
O2—C4—C3—C2 | −1.27 (19) | C6—C7—C8—C9 | 179.36 (16) |
C5—C4—C3—C2 | −179.44 (12) | C8—C7—C12—C11 | −0.6 (2) |
O2—C4—C3—C13 | −178.58 (12) | C6—C7—C12—C11 | −179.83 (16) |
C5—C4—C3—C13 | 3.2 (2) | C7—C8—C9—C10 | 1.2 (3) |
O1—C2—C3—C4 | −0.7 (2) | C11—C10—C9—C8 | −2.1 (3) |
C1—C2—C3—C4 | −177.69 (13) | C9—C10—C11—C12 | 1.6 (3) |
O1—C2—C3—C13 | 176.65 (12) | C7—C12—C11—C10 | −0.3 (3) |
C1—C2—C3—C13 | −0.4 (2) | C13—O4—C14—C15 | −177.93 (12) |
C7—C6—C5—C4 | 178.81 (12) | C14—O4—C13—O3 | 10.01 (19) |
O2—C4—C5—C6 | 4.40 (19) | C14—O4—C13—C3 | −170.26 (11) |
C3—C4—C5—C6 | −177.34 (13) | C4—C3—C13—O3 | 42.0 (2) |
C5—C6—C7—C8 | −5.0 (2) | C2—C3—C13—O3 | −135.22 (14) |
C5—C6—C7—C12 | 174.17 (15) | C4—C3—C13—O4 | −137.70 (12) |
C12—C7—C8—C9 | 0.2 (3) | C2—C3—C13—O4 | 45.06 (17) |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O1 | 1.03 (3) | 1.45 (3) | 2.427 (2) | 157 (2) |
C10—H10···O2i | 0.98 (2) | 2.79 (2) | 3.707 (2) | 155 (2) |
C11—H11···O3ii | 0.97 (2) | 2.52 (2) | 3.349 (2) | 143 (2) |
Symmetry codes: (i) −x+3/2, y+1/2, z−1/2; (ii) −x+3/2, y+1/2, z+1/2. |
Experimental details
Crystal data | |
Chemical formula | C15H16O4 |
Mr | 260.28 |
Crystal system, space group | Orthorhombic, Pnn2 |
Temperature (K) | 123 |
a, b, c (Å) | 21.698 (4), 7.3390 (15), 8.6380 (17) |
V (Å3) | 1375.5 (5) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.4 × 0.3 × 0.3 |
Data collection | |
Diffractometer | Bruker SMART diffractometer |
Absorption correction | Empirical (using intensity measurements) (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.964, 0.973 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 13464, 2813, 2703 |
Rint | 0.024 |
(sin θ/λ)max (Å−1) | 0.625 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.031, 0.079, 1.06 |
No. of reflections | 2813 |
No. of parameters | 236 |
No. of restraints | 1 |
H-atom treatment | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.13, −0.16 |
Computer programs: SMART (Bruker, 1998), SMART, SAINT (Bruker, 1998), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), SHELXL97.
C4—O2 | 1.3081 (18) | C3—C13 | 1.4882 (18) |
C4—C3 | 1.4066 (19) | C6—C5 | 1.3398 (19) |
C4—C5 | 1.4601 (18) | C6—C7 | 1.466 (2) |
O1—C2 | 1.2643 (18) | O4—C13 | 1.3390 (16) |
O1—H2 | 1.45 (3) | O4—C14 | 1.4594 (18) |
O2—H2 | 1.03 (3) | O3—C13 | 1.2091 (17) |
C2—C3 | 1.4481 (19) | C14—C15 | 1.505 (2) |
C2—C1 | 1.498 (2) | ||
O2—C4—C3 | 120.67 (13) | C2—C3—C13 | 120.94 (12) |
O2—C4—C5 | 116.06 (12) | C5—C6—C7 | 126.44 (14) |
C3—C4—C5 | 123.25 (14) | C6—C5—C4 | 121.24 (13) |
C2—O1—H2 | 101.0 (11) | C8—C7—C6 | 122.41 (14) |
C4—O2—H2 | 103.2 (15) | C12—C7—C6 | 119.42 (14) |
O1—C2—C3 | 119.92 (14) | C13—O4—C14 | 116.54 (11) |
O1—C2—C1 | 117.34 (13) | O4—C14—C15 | 106.98 (13) |
C3—C2—C1 | 122.68 (14) | O3—C13—O4 | 123.01 (13) |
C4—C3—C2 | 118.30 (14) | O3—C13—C3 | 125.16 (13) |
C4—C3—C13 | 120.71 (12) | O4—C13—C3 | 111.83 (11) |
C4—C3—C13—O3 | 42.0 (2) | C4—C3—C13—O4 | −137.70 (12) |
C2—C3—C13—O3 | −135.22 (14) | C2—C3—C13—O4 | 45.06 (17) |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O1 | 1.03 (3) | 1.45 (3) | 2.427 (2) | 157 (2) |
C10—H10···O2i | 0.98 (2) | 2.79 (2) | 3.707 (2) | 155 (2) |
C11—H11···O3ii | 0.97 (2) | 2.52 (2) | 3.349 (2) | 143 (2) |
Symmetry codes: (i) −x+3/2, y+1/2, z−1/2; (ii) −x+3/2, y+1/2, z+1/2. |
The reaction of the boron complex of 2-trans-cinnamonyl acetic ethyl ester (CAA) with aromatic or heteroaromatic aldehydes and subsequent acid hydrolysis opened a new procedure for the synthesis of deep coloured β,β'-tricarbonyl compounds (Arrieta, 1993). This was an extension of the Pabon reaction (Pabon, 1964) for β-diketo esters and showed the ability and reactivity of the acetyl group for further condensation reactions. The β,β'-tricarbonyl compounds are interesting molecules also because of the possibility of adopting several different enol tautomer conformations. Thus, the title compound, (4rs), may exist in seven different forms.
Earlier IR spectrographic studies of the title compound (Arrieta et al., 1988) have shown that the carboethoxy group is peripheral and non-coordinating in copper complexes. Moreover, a recent 13C NMR study (Emelina et al., 1999) has indicated that the CAA molecule adopt the (1a) conformation in solution. The postulated chelat bond between the acetyl group and the cinnamonyl group in CAA has been reinvestigated in the solid solution (KBr) and in liquid solution (CHCl3) using IR and NMR spectroscopy. The IR region at 1310 cm-1 in CCl4 was assigned by Forsen (Forsen et al., 1959) to the cinnamonyl group, and probably refers to the group C(═O)CH═CH. The 13C NMR spectrum in the same solvent confirms, in accord with recent studies (Emelina et al., 1999), the absence of any other tautomers than (1a). The 1H NMR spectrum in CDCl3 shows only one signal for the enol proton (17.60 p.p.m.) in contrast to 2-acetylbenzoylacetates where at least two are found (Sicker et al., 1988). The reason for this seems unclear and NMR studies of such compounds are in progress (Arrieta & Radeglia, 2001). The structural study of 3-benzoyl-6-phenyl-hex-5-ene-2,4-dione (Arrieta et al., 1995), which differ from the title compound only in the exchange of the benzoyl group with a carbethoxy group, showed that the conformation adopted in the crystal phase is that where the longest possible conjugation in the molecule, as well as the positioning of the enol H atom at the oxygen closest to the most electronegative substituent, is simultaneously satisfied. The result of the present investigation shows that the 6-phenylhex-5-ene-2,4-dione parts of the two molecules have virtually identical measures in bond lengths, angles and conformation. This supports the idea that the conjugation and the electronegativities of the enol ring substituents are determining factors for the conformation and the position of the enol H atom. It may be seen from the Scheme, that only conformation (1a) fulfil the two criteria. In the crystal structure of CAA, the molecules related by rotation axes are stacked along the b axis with overlapping enol and carboxylate groups. The distances between the planes through these two groups are 3.423 (2) and 3.391 (2) Å, respectively. The closest contacts between molecules occur through C—H···O interactions. The geometry of these interactions (Table 2) and the fact that such interactions are repeatedly found in the crystal structures of this group of molecules (Mostad, 1994; Arrieta et al., 1995, 2000), indicate that they may be considered as examples of weak hydrogen bonds (Desiraju & Steiner, 1999). A drawing of the molecule with the numbering of the atoms and their vibrational elipsoids is given in Fig. 1. The packing of the molecules is displayed in Fig. 2.